The tertiary and quaternary structures of an immunoglobulin molecule is being investigated using single crystal x-ray diffraction analysis of its crystals. Phases calculated using the multiple isomorphous replacement method have been used to calculate the electron density map at a resolution of 4A. Structures of several of the known fragments of the immunoglobulin molecule have been rotated, translated and superposed on the electron density map and the best possible fit has been obtained. This structure is now being refined using a stereochemically constrained least squares procedure. A new electron density map that will combine MIR phases and partial structure information will be calculated and the known partial sequence will be fitted on this map. The effect of binding various ligands to the immunoglobulin molecule at the antigen binding site will be investigated.